Pixel structure and electric device

ABSTRACT

A pixel structure of an image sensor is provided and includes following units. A crystalline layer of a first doping type is formed on a substrate. A photodiode region is formed in the crystalline layer. A gate of a source follower transistor is formed on a top surface of the crystalline layer. A reset gate is formed on the top surface of the crystalline layer. A doped region of a second doping type is formed in the crystalline layer and formed between the reset gate and the gate of the source follower. The first doping type is different from the second doping type, and the photodiode region is connected to the doped region under the top surface of the crystalline layer as an anti-blooming path.

BACKGROUND Field of Invention

The present invention relates to a pixel structure having ananti-blooming path.

Description of Related Art

A complementary metal-oxide-semiconductor (CMOS) image sensor has beenwidely applied to mobile applications. The CMOS image sensor may beapplied to other applications such as automotive and security.Requirements for the automotive and security applications are quitedifferent from that for the mobile applications. For example, bloomingis highly undesirable in automotive and surveillance application.Blooming happens when a pixel is filled up with photo carriers and canno longer collect more electron/hole pairs during pixel exposure. Abright pixel will spreads to several other pixels in the neighboringregion.

The road scene, especially at night, has a high dynamic range. The CMOSimage sensor is thus required to have a good blooming control atultra-bright region in order to ensure that the neighboring dimly litregions are not washed out by the blooming charges. Otherwise, manydetails get lost and it is difficult to extract the information from thescene. Moreover, at high temperature operation such as in a car, a hotpixel could be filled up by dark current even in the dark. The adjacentgood pixels will become hot by receiving the blooming charges.

For the reason that conventional CMOS image sensors could noteffectively solve blooming problem, a need has thus arisen to propose anovel CMOS image sensor with improved anti-blooming.

SUMMARY

Embodiments of the invention provide a pixel structure includingfollowing units. A crystalline layer of a first doping type is formed ona substrate. A photodiode region is formed in the crystalline layer. Agate of a source follower transistor is formed on a top surface of thecrystalline layer. A reset gate is formed on the top surface of thecrystalline layer. A doped region of a second doping type is formed inthe crystalline layer and formed between the reset gate and the gate ofthe source follower. The first doping type is different from the seconddoping type, and the photodiode region is connected to the doped regionunder the top surface of the crystalline layer as an anti-blooming path.

In some embodiments, the pixel structure further includes an isolationstructure extending from the top surface of the crystalline layer andformed between the photodiode region and the doped region. Thephotodiode region is connected to the doped region under the isolationstructure.

In some embodiments, the photodiode region has a first portion of thesecond doping type that extends toward the doped region under theisolation structure. The doped region has a second portion of the seconddoping type that extends toward the photodiode region under theisolation structure, and the first portion is connected to the secondportion.

In some embodiments, the doped region includes a highly doped region ofthe second doping type, a lightly doped drain of the second doping typelocated under the highly doped region, and the second portion whichextends toward the photodiode region.

In some embodiments, a doping concentration of the second portion isless than a doping concentration of the highly doped drain.

In some embodiments, the pixel structure further includes a well regionof the first doping type formed in the crystalline layer. The wellregion has a notch to at least partially surround the first portion andthe second portion.

In some embodiments, the well region is vertically spaced from aconnection interface between the first portion and the second portion bya distance.

In some embodiments, the width of the notch is shorter than a distancebetween the reset gate and the gate of the source follower.

In some embodiments, the first doping type is P-type, and the seconddoping type is N-type.

In some embodiments, the doped region is connected to a power supply.

From another aspect, an electrical device including the aforementionedpixel structure is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the followingdetailed description of the embodiment, with reference made to theaccompanying drawings as follows.

FIG. 1 is a schematic pixel circuit diagram of an image sensor inaccordance with an embodiment.

FIG. 2A is a top view of the pixel structure of the image sensor inaccordance with an embodiment.

FIG. 2B is a cross-sectional view along a section line AA′ of FIG. 2A.

FIG. 3A is a cross-sectional view of the partial pixel structure inaccordance with an embodiment.

FIG. 3B is a schematic top view of the partial pixel structure along asection line BB′ of FIG. 3A.

FIG. 3C is a schematic top view of the partial pixel structure along asection line CC′ of FIG. 3A.

FIG. 3D is a schematic top view of the partial pixel structure along asection line DD′ of FIG. 3A.

FIG. 4 is a top view of the pixel structure of image sensor inaccordance with an embodiment.

FIGS. 5A, 5B, and 5C are cross-sectional view of the partial pixelstructure in accordance with some embodiments.

DETAILED DESCRIPTION

Specific embodiments of the present invention are further described indetail below with reference to the accompanying drawings, however, theembodiments described are not intended to limit the present inventionand it is not intended for the description of operation to limit theorder of implementation. Moreover, any device with equivalent functionsthat is produced from a structure formed by a recombination of elementsshall fall within the scope of the present invention. Additionally, thedrawings are only illustrative and are not drawn to actual size.

FIG. 1 is a schematic pixel circuit diagram of an image sensor inaccordance with an embodiment. Referring to FIG. 1, an image sensor 100may be applied to a front side illumination (FSI) image sensor or a backside illumination (BSI) image sensor. The image sensor 100 includesmultiple pixels, and each of the pixels includes a photodiode 110, atransfer transistor TX, a reset transistor RES, a source follower SF,and a select transistor SEL. The photodiode 110 has an anodeelectrically connected to the ground, and a cathode electricallyconnected to a first terminal of the transfer transistor TX. A secondterminal of the transfer transistor TX is electrically connected to afirst terminal of the reset transistor RES and a gate of the sourcefollower SF. A second terminal of the reset transistor RES iselectrically connected to a power supply VDD. The source follower SF hasa first terminal electrically connected to the power supply VDD, and asecond terminal electrically connected to a first terminal of the selecttransistor SEL. A second terminal of the select transistor SEL iselectrically connected to a bias 130 and outputs to a sensing circuit140. In the embodiment, an anti-blooming path 120 is provided from thephotodiode 110 to the power supply VDD. One pixel structure will bedescribed in detail below.

FIG. 2A is a top view of the pixel structure of the image sensor inaccordance with an embodiment. FIG. 2B is a cross-sectional view along asection line AA′ of FIG. 2A. Referring to FIG. 2A and FIG. 2B, the imagesensor 100 includes a substrate 201 of a first doping type (e.g.P-type). A crystalline layer 202 of the first doping type, such asP-type epitaxial layer or P-epi, is formed on the substrate 201.Photodiode regions 203, 214 and a well region PW of the first dopingtype are formed in the crystalline layer 202. The photodiode region 203belongs to one of the pixels of the image sensor 100, and the photodioderegion 214 belongs to an adjacent pixel. A surface pinning layer 210 anda gate insulation layer 211 are formed on a top surface 202 a of thecrystalline layer 202. For example, the gate insulation layer 211includes oxide. Isolation structures 212, 213 are formed in thecrystalline layer 202, in which the isolation structure 212 is formedbetween the photodiode region 203 and the photodiode region 214. Forexample, the isolation structures 212, 213 are shallow trench isolations(STI). In some embodiments, the isolation structure 212 further includesa deep well 215 of the first doping type.

A gate TX_G (also referred to a transfer gate) of the transfertransistor TX, a gate RES_G (also referred to a reset gate) of the resettransistor RES, a gate SF_G of the source follower SF, and a gate SEL_G(also referred to a select gate) of the select transistor SEL are formedon the gate insulation layer 211 (these gates are not shown in FIG. 2B).The gate TX_G covers a portion of the photodiode region 203. Dopedregion 204, 206, and 207 are formed in the crystalline layer 202. Thedoped region 204 is formed between the transfer gate TX_G and the resetgate RES_G as source/drain of the transfer transistor TX and the resettransistor RES. The doped region 204 is also electrically connected tothe gate SF_G through a conductive structure 205. The doped region 206is formed between the reset gate RES_G and the gate SF_G as source/drainof the reset transistor RES and the source follower SF. The doped region207 is formed between the gate SF_G and the select gate SEL_G assource/drain of the source follower SF and the select transistor SEL.The doped regions 204, 206, and 207 have a second doping type (e.g.N-type) which is different from the first doping type.

The photodiode region 203 is connected to the doped region 206 under thetop surface 202 a of the crystalline layer 202 so as to provide theanti-blooming path 120. In detail, the isolation structure 213 extendsfrom the top surface 202 a of the crystalline layer 202, and is formedbetween the photodiode region 203 and the doped region 206. Thephotodiode region 203 has a first portion 221 of the second doping type(e.g. N-type) that extends toward the doped region 206 under theisolation structure 213. In addition, the doped region 206 has a secondportion 222 of the second doping type that extends toward the photodioderegion 203 under the isolation structure 213. The first portion 221 isconnected to the second portion 222 under the isolation structure 213.That is, the photodiode region 203 is connected to the doped region 206under the isolation structure 213. Note that the first portion 221 andthe second portion 222 are illustrated by dotted line in FIG. 2A becausethey are covered by the isolation structure 213.

In some embodiments, the doped region 206 includes a highly doped region224, a lightly doped drain 223 under the highly doped region 224, andthe second portion 222 in which all of these 222/223/224 have the seconddoping type. In some embodiments, the second portion 222 is formed bythermal diffusion so that the doping concentration of the second portion222 is less than that of the highly doped region 224, and the dopingconcentration of the second portion 222 progressively decreases towardthe photodiode region 203. Therefore, it should be appreciated thatthere is no clear boundary between the lightly doped drain 223 and thesecond portion 222 in some embodiments. Because the first portion 221and the second portion 222 have the same doping type and are connectedto the each other, the barrier between the photodiode region 203 anddoped region 206 is lower than that between the photodiode region 203and the photodiode region 214. Referring to FIG. 1 and FIG. 2B, thedoped region 206 is connected to the power supply VDD. When thephotodiode region 203 is overexposed, extra photo electrons flow intothe power supply VDD through the anti-blooming path 120 instead offlowing into the adjacent photodiode region 214. In some embodiments,the depth of the highly doped region 224 may be adjusted in which deeperdepth leads to lower barrier between the first portion 221 and thesecond portion 222. In some embodiments, the depth of the isolationstructure 213 may be adjusted in which deeper depth leads to higherbarrier between the first portion 221 and the second portion 222.

FIG. 3A is a cross-sectional view of the partial pixel structure inaccordance with an embodiment. FIG. 3A is identical to FIG. 2B withdifferent references for illustration. FIG. 3B is a schematic top viewof the partial pixel structure along a section line BB′ of FIG. 3A. FIG.3C is a schematic top view of the partial pixel structure along asection line CC′ of FIG. 3A. FIG. 3D is a schematic top view of thepartial pixel structure along a section line DD′ of FIG. 3A. Referringto FIG. 3A, in some embodiments, the well region PW has a notch for atleast partially surrounding the first portion 221 and the second portion222. In detail, the space under the isolation structure 213 is occupiedby the well region PW, similar to the deep well 215, in the prior art.However, in the embodiment, the well region PW has a notch 320 withheight H. The well region PW is vertically spaced from a connectioninterface 310 between the first portion 221 and the second portion 222by a distance D1. Referring to FIG. 3B, the notch 320 of the well regionPW partially surrounds the first portion 221 and second portion 222 fromthree sides. The notch 320 has a width W and a length L. Referring toFIG. 3C, the notch 320 of the well region PW is filled with thecrystalline layer 202. Referring to FIG. 3D, the notch 320 is not seenin FIG. 3D.

Note that the height H, width W, and length L of the notch 320, and thedistance D1 may be adjusted based on requirements. In general, when theheight H, the width W, the length L, and the distance D1 get larger, thebarrier formed by the first portion 221 and the second portion 222 getslower so as to generate more efficient anti-blooming path 120.

FIG. 4 is a top view of the pixel structure of the image sensor inaccordance with an embodiment. FIG. 4 is basically equal to FIG. 2A, butthe well region PW is shown in FIG. 4 for illustration. Note that thewell region PW is embedded in the crystalline layer 202, and thereforethe well region PW cannot be “seen” from the top view. In FIG. 4, thenotch 320 of the well region PW has the width W and the length L. Thewidth W is shorter than a distance D2 between the reset gate RES_G andthe gate SF_G. However, the notch 320 may extend into a channel regionbetween the reset gate RES_G and the gate SF_G if more anti-bloomingstrength is needed, as long as the operation of the reset gate RES_G andthe gate SF_G is not disturbed.

In some embodiments, in addition to the first portion 221 and the secondportion 222 created mainly by thermal diffusions from the photodioderegion 203 and the doped region 206, an implant region of second dopingtype may be formed in the crystalline layer 202 for adjusting theanti-blooming strength. For example, referring to FIG. 5A, an implantregion 510 is formed under the isolation structure 213, and formedbetween the photodiode region 203 and the doped region 206. The implantregion 510 is also connected to the photodiode region 203 and the dopedregion 206 to provide an anti-blooming path. Referring to FIG. 5B, animplant region 520 is formed under the isolation structure 213, but whatis different from FIG. 5A is that the implant region 520 extends towardthe photodiode region 203 so could be shared with one photodiode region203 implant. Referring to FIG. 5C, an implant region 530 is partiallyformed in the notch 320, and connected to the doped region 206 and thewell region PW. The implant region 530 may be an additional element ofthe doped region 206. Note that the lengths, widths, heights, and thelocations of the implant regions 510, 520, and 530 may be adjusted basedon requirement, which is not limited in the invention.

From another aspect, an electrical device is provided in someembodiments. The electrical device includes the aforementioned imagesensor 100 with the aforementioned pixel structure. The electricaldevice may be a smart phone, any type of computer, digital camera, etc.that is not limited in the invention.

In some embodiments, the first doping type may be N-type, and the seconddoping type may be P-type.

Although the present invention has been described in considerable detailwith reference to certain embodiments thereof, other embodiments arepossible. Therefore, the spirit and scope of the appended claims shouldnot be limited to the description of the embodiments contained herein.It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims.

1. A pixel structure, comprising: a substrate; a crystalline layer of afirst doping type formed on the substrate; a photodiode region formed inthe crystalline layer; a gate of a source follower transistor formed ona top surface of the crystalline layer; a reset gate formed on the topsurface of the crystalline layer; and a doped region of a second dopingtype formed in the crystalline layer and formed between the reset gateand the gate of the source follower, wherein no gate portion of thepixel structure is formed directly above the doped region, wherein thefirst doping type is different from the second doping type, and thephotodiode region is connected to the doped region under the top surfaceof the crystalline layer as an anti-blooming path.
 2. The pixelstructure of claim 1, further comprising: an isolation structureextending from the top surface of the crystalline layer and formedbetween the photodiode region and the doped region, wherein thephotodiode region is connected to the doped region under the isolationstructure.
 3. The pixel structure of claim 2, wherein the photodioderegion has a first portion of the second doping type that extends towardthe doped region under the isolation structure, and the doped region hasa second portion of the second doping type that extends toward thephotodiode region under the isolation structure, and the first portionis connected to the second portion at a connection interface, whereinthe connection interface is located directly under the isolationstructure.
 4. The pixel structure of claim 3, wherein the doped regioncomprises a highly doped region of the second doping type, a lightlydoped drain of the second doping type located under the highly dopedregion, and the second portion which extends toward the photodioderegion.
 5. The pixel structure of claim 4, wherein a dopingconcentration of the second portion is less than a doping concentrationof the highly doped drain.
 6. The pixel structure of claim 3, furthercomprising: a well region of the first doping type formed in thecrystalline layer, wherein the well region has a notch to at leastpartially surround the first portion and the second portion.
 7. Thepixel structure of claim 6, wherein the well region is vertically spacedfrom the connection interface between the first portion and the secondportion by a distance.
 8. The pixel structure of claim 6, wherein thewidth of the notch is shorter than a distance between the reset gate andthe gate of the source follower.
 9. The pixel structure of claim 1,wherein the first doping type is P-type, and the second doping type isN-type.
 10. The pixel structure of claim 1, wherein the doped region isconnected to a power supply.
 11. An electrical device comprising: apixel structure comprising: a substrate; a crystalline layer of a firstdoping type formed on the substrate; a photodiode region formed in thecrystalline layer; a gate of a source follower transistor formed on atop surface of the crystalline layer; a reset gate formed on the topsurface of the crystalline layer; and a doped region of a second dopingtype formed in the crystalline layer and formed between the reset gateand the gate of the source follower, wherein no gate portion of thepixel structure is formed directly above the doped region, wherein thefirst doping type is different from the second doping type, and thephotodiode region is connected to the doped region under the top surfaceof the crystalline layer as an anti-blooming path.
 12. The electricaldevice of claim 11, wherein the pixel structure further comprises: anisolation structure extending from the top surface of the crystallinelayer and formed between the photodiode region and the doped region,wherein the photodiode region is connected to the doped region under theisolation structure.
 13. The electrical device of claim 12, wherein thephotodiode region has a first portion of the second doping type thatextends toward the doped region under the isolation structure, and thedoped region has a second portion of the second doping type that extendstoward the photodiode region under the isolation structure, and thefirst portion is connected to the second portion at a connectioninterface, wherein the connection interface is located directly underthe isolation structure.
 14. The electrical device of claim 13, whereinthe doped region comprises a highly doped region of the second dopingtype, a lightly doped drain of the second doping type located under thehighly doped region, and the second portion which extends toward thephotodiode region.
 15. The electrical device of claim 14, wherein adoping concentration of the second portion is less than a dopingconcentration of the highly doped drain.
 16. The electrical device ofclaim 13, wherein the pixel structure further comprises: a well regionof the first doping type formed in the crystalline layer, wherein thewell region has a notch to at least partially surround the first portionand the second portion.
 17. The electrical device of claim 16, whereinthe well region is vertically spaced from the connection interfacebetween the first portion and the second portion by a distance.
 18. Theelectrical device of claim 16, wherein the width of the notch is shorterthan a distance between the reset gate and the gate of the sourcefollower.
 19. The electrical device of claim 11, wherein the firstdoping type is P-type, and the second doping type is N-type.
 20. Theelectrical device of claim 11, wherein the doped region is connected toa power supply.